Pteraeolidia ianthina (Angas,1864)
blue dragon
Pteraeolidia ianthina

sub-classs Opisthobranchia > order Nudibranchia > sub-order Aeolidina > family Facelinidae/Glaucidae

Top left - Unlike most nudibranchs, Pteraeolidia ianthina, usually form groups of four to six individuals to protect the eggs until they hatch. Unusually, these eggs had been abandoned
Botom left - Blue dragons eat hydroids, preferably large solitary ones such as Ralpharia magnifica. They can store undigested nematocysts in the cerata on their backs, so they can sting.
RIght - Blue dragons are variable in shape and colour. Juveniles are white, adults purple to brown. The colour comes from zooxnathellae which live symbiotically within the animal. The animal can go for some time without feeding, presumably by obtaining nourishment from their on-board solar collectors.

Distribution =   Widespread Indo-west Pacific
Max size =  at least 15cms

ID by xx     more info at Sea Slug Forum     more pics at nudipixel